Cartesian modules in small categories

In this note we extend the main results of [E. Enochs and S. Estrada, Relative homological algebra in the category of quasi-coherent sheaves. Adv. in Math. 194(2005), 284-295] to the category of cartesian modules over a flat presheaf of rings and on an arbitrary small category. This provides with new applications of that paper to the categories of quasi-coherent sheaves on an Artin stack or on a Deligne-Mumford stack.